The present application is based upon Japanese application nos. 2003-050016 filed Feb. 26, 2003 and 2003-050124 filed Feb. 26, 2003 and claims priority to U.S. application Ser. No. 10/785,029 filed Feb. 25, 2004, the disclosure of which is herein incorporated by reference.
The present invention relates to a common-mode choke coil, a method for producing the same, and a common-mode choke coil array. Particularly it relates to a filter used for suppressing a common-mode current as a cause of electromagnetic interference becoming an issue in a balanced transmission system, and a method for producing the filter.
A laminated type choke coil is heretofore known as a chip common-mode choke coil. This component has a structure in which a first coil magnetic sheet having a coil conductor pattern formed on a surface of a sheet of magnetic substance such as ferrite to form a first coil and a second coil magnetic sheet formed in the same manner as the first coil magnetic sheet are laminated alternately.
A common-mode choke coil described in Patent Document JP-A-8-203737 is known as a choke coil produced by a thin-film process. This component has a structure in which: a lead-out electrode is formed on a magnetic substrate by a thin-film process; an electrically insulating layer, a first coil conductor, an electrically insulating layer, a second coil conductor and an electrically insulating layer are then formed successively thereon by a thin-film process; and another magnetic substrate is put on an upper surface of the resulting laminate.
Patent Document JP-A-11-54326 has described a common-mode choke coil produced by a thin-film process. In this document, central and outer circumferential portions of each electrically insulating layer produced by the thin-film process are etched (developed) in order to improve the degree of magnetic coupling between coils and increase common impedance. The upper magnetic substrate is bonded by a resin as a mixture of an electrically insulating material and magnetic powder to thereby form a closed magnetic circuit structure.
In the background-art laminated type choke coil, the sheet of magnetic substance is interposed between the first and second coil conductors. When this choke coil is used as a common-mode choke coil, magnetic coupling between two coils is lowered. This becomes a characteristic issue.
In the common-mode choke coil produced by the thin-film process described in Patent Document JP-A-8-203737, the first and second coil conductors are sandwiched between the upper and lower magnetic substrates. Because this structure cannot be provided as a closed magnetic circuit structure, there is a problem in the degree of magnetic coupling and common impedance (it is difficult to improve the degree of magnetic coupling and common impedance).
In the thin-film process described in Patent Document JP-A-11-54326, the aforementioned problem is solved because the closed magnetic circuit structure is formed. The magnetic power-containing resin is however low in adhesive property. There is a problem in adhesion of the magnetic substrate.
As measures to solve this problem, it is possible to bond the magnetic substrate by an adhesive agent after the magnetic powder-containing resin is applied on the magnetic substrate-bonding side of an electrically insulating layer with a built-in coil conductor and cured. The magnetic powder-containing resin is however shrunk when cured. Because the magnetic powder-containing resin embedded in the etching portion (the place where the central and outer circumferential portions of the electrically insulating layer are etched to form a closed magnetic circuit structure) of the electrically insulating layer is shrunk, only the adhesive layer (non-magnetic) as the upper portion of the electrically insulating layer is thickened. Accordingly, it is impossible to obtain a sufficient effect in increasing common impedance.
In the latest balanced transmission line, it is however necessary to perform high-speed transmission of a transmission signal with a frequency of the order of GHz. The thin-film type common-mode choke coil cannot meet such high-speed transmission.
Measures to meet high-speed transmission are suggested as follows.
Firstly, matching of characteristic impedance is suggested. The fact that matching is important is a commonly known fact on a transmission circuit.
Secondly, greater improvement in magnetic coupling is suggested. This improvement is however structurally limited.
Thirdly, reduction in capacitance between conductors is suggested. If the space between the conductors is merely increased in order to reduce such capacitance, increase in size cannot be avoided.